On the structural principle of shielded pumps and the circulation path of the circulating fluid

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I referred to and summarized the structural principles of shielded pumps as well as the pathways of the circulating fluid discussed in the forum, with the intention of drafting a maintenance plan for these pumps. The following text is part of that maintenance plan; once it’s completed, I will upload it and share it here. The reason for posting this now is to ask experts from various fields to check if there are any mistakes in what I’ve written and to provide suggestions so that I can make the necessary corrections. Thank you. Principle: 1. A shielded pump is a type of sealless pump, where both the pump itself and the drive motor are enclosed within a pressure vessel filled with the medium to be pumped. This pressure vessel has only static seals, and an electric coil is used to generate a rotating magnetic field that drives the rotor. This design eliminates the rotating shaft sealing mechanism found in conventional centrifugal pumps, thereby achieving complete leaklessness. 2. A canned pump combines the pump and the motor together; the rotor of the motor and the impeller of the pump are mounted on the same shaft. Stator and rotor shielding sleeves made of non-magnetic, corrosion-resistant metal sheets are used to separate the rotor from the stator, sealing each of them so that they do not come into contact with the liquid medium being transported. This prevents the motor’s core and windings from being corroded and ensures that the stator windings maintain good insulation properties. The rotor rotates within the medium being transported, with power being transmitted to it through the stator’s magnetic field. The two ends of the rotor core are supported by graphite bearings, and an impeller is installed at the front end of the rotor, resulting in a shielded pump without an axis seal. The axial thrust generated by the impeller acts on the front and rear thrust plates, with the liquid being transported serving as the lubricant. 3. The wear condition of the bearings and thrust discs is crucial for reliable operation. To monitor bearing wear, mechanical or electromagnetic bearing monitors (TRG) are generally installed. When the wear of the bearing exceeds the specified allowable value, the pointer on the monitor dial will move from the green zone to the yellow or red zone, indicating an \"alarm\". Operation must be stopped immediately when pointing at the yellow and red zones for inspection. If the wear level of the bearing has exceeded the limit value, a new graphite bearing should be installed; otherwise, it may lead to friction between the stator and rotor shielding sleeves, eventually causing damage to these sleeves. This can result in the liquid medium penetrating the stator windings and other components, thereby damaging the motor. Characteristics of the circulating fluid: The main circulation methods of the circulating fluid within a shielded pump are internal shaft circulation, external shaft circulation, and reverse circulation. 1. Internal shaft circulation (basic type): Flow direction of the circulating fluid: Pump outlet → Circulation pipe filter → Circulation pipe → Rear end cover (bearing housing at the non-driving end) → Gap between the rear bearing (bearing at the non-driving end) and the rear shaft sleeve (shaft sleeve at the non-driving end) → Axial gap between the rear thrust disc and the rear bearing → Gap between the stator shield and the rotor shield → Axial gap between the front thrust disc and the front bearing → Gap between the front bearing (bearing at the driving end) and the front shaft sleeve (shaft sleeve at the driving end) → Impeller balance hole → Impeller inlet. 2. External shaft circulation (basic type): Flow direction of the circulating liquid: pump inlet → impeller → a portion through the holes in the front cover (or the holes in the connecting element) → the gap between the stator shield and the rotor shield → the axial gap between the rear thrust disc and the rear bearing → the gap between the rear bearing (the non-driving-end bearing) and the rear shaft sleeve (the non-driving-end bearing sleeve) → the through-hole in the bolt of the rear (non-driving-end) shaft head → the internal shaft hole → impeller. Another small portion flows from the hole in the front cover (or the hole in the connecting element) → the axial gap between the front thrust disc and the front bearing → the gap between the front bearing (the drive-end bearing) and the front shaft sleeve (the drive-end shaft sleeve) → the impeller balance hole → the impeller inlet. 3. When volatile liquids are used in applications with a low NPSH margin, the circulating fluid absorbs heat from the motor as well as heat generated by bearing friction, which causes the temperature of the liquid to rise. If the basic design is used, cavitation can occur easily when the circulating fluid returns to the inlet of the impeller, preventing the pump from functioning properly. To address this issue, the circulating fluid is directed back to the top of the liquid supply tank instead of to the impeller inlet, thereby avoiding cavitation. Reverse circulation (reverse-cycle type): Flow direction of the circulating liquid: from the impeller outlet → a portion through the holes in the front cover (or the holes in the connecting element) → the gap between the stator shield and the rotor shield → the axial gap between the rear thrust disc and the rear bearing → the gap between the rear bearing (the non-driving-end bearing) and the rear shaft sleeve (the non-driving-end bearing sleeve) → the rear cover (the housing for the non-driving-end bearing) → back to the top of the liquid inlet tank. Another small portion passes through the hole in the front cover (or the hole in the connecting element) → the axial gap between the front thrust disc and the front bearing → the gap between the front bearing (the drive-end bearing) and the front shaft sleeve (the drive-end shaft sleeve) → the gap between the front cover and the hub of the impeller → the inlet of the impeller.